Ideally, RF power amplifiers would act linearly, faithfully reproducing an amplified RF signal at their output with no distortion. Requirements for efficiency, however, can lead to operating amplifiers close to saturation, where non-linearities create unwanted IMD. IMD products may cause interference, disrupting the proper transmission and reception of RF signals, particularly in adjacent channels. Numerous techniques have been developed to reduce IMD products from amplified RF signals, including feed forward, predistortion, and linear amplification with non-linear components (LINC).
Recent surges in demand for wireless solutions have led to new frequency bands to increase capacity, such as, for example, the Universal Mobile Telecommunications System (UMTS) developed by the European Telecommunications Standard Institute for delivering 3G (third generation) services. Modern transmission protocols, such as UMTS, demand high linearity to prevent radio frequency energy in one band from spilling over and interfering with other proximate channels, but often have high Peak-to-Average Power Ratio (PAR) carrier signals that make efficient linear amplifiers difficult to design. This energy leakage can undesirably degrade the signal-to-noise (SNR) ratio or bit-error rate (BER) of the proximate frequency channels.
In practice, it is very difficult and often unnecessary to eliminate completely all IMD products for a selected center frequency. A certain tolerable level of IMD products is acceptable. When the terms “eliminate” or “reduce” are used herein with reference to the IMD products, it is understood that the IMD products should be suppressed below a certain tolerable level, even though they may not be entirely eliminated.
One common technique to reduce IMD to acceptable levels is feed forward correction, whereby the IMD products are isolated and manipulated so that at the final summing point the IMD products substantially cancel out. However, the input signal pattern can be unpredictable, often making the IMD products difficult to locate. One way to address this problem is to inject an artificial signal, conventionally called a pilot tone or pilot signal, to simulate the unwanted distortion to be removed. At the output, a pilot signal receiver detects the simulated distortion, and the amplifier is aligned in accordance with a signal representative of the pilot signal receiver output. Significantly, these pilot signal receivers do not detect and measure the actual non-linear distortion components. Instead, they detect and measure the simulated distortion based on an injected pilot signal so that, at the final summing point, the simulated distortion is canceled out with the intent that IMD will also cancel out, leaving only the amplified carrier signals.
Some pilot tone systems inject the pilot signal into the main signal before the carriers are amplified; others inject the pilot signal after the carriers have been amplified. In either case, the distortion products contain “artificial” distortion products in addition to the non-linear distortion products created by the power amplifier. As a result pilot tone systems suffer from several drawbacks. First, pilot tone systems do not actually detect and eliminate the actual distortion produced by the power amplifier. Because they detect distortion created by an artificially injected signal and not the actual distortion created by the power amplifier, the actual distortion may not be entirely cancelled and the artificial distortion may leak into the output. Moreover, circuit complexity, size, and cost are increased because the pilot tone circuit must include a pilot signal generator, and a pilot signal injector, among other things. In addition, the pilot signal receiver may need to be tightly synchronized with the transmitter to obtain optimum cancellation of the distortion products generated by the pilot signal and the power amplifier.
In another technique, the locations of the distortion products can be calculated without the use of a pilot tone or signal. In one such technique, an amplified WCDMA carrier containing both in-band frequency components and undesired spectral regrowth components is downconverted to baseband, digitized by an analog-to-digital converter, and then spectrally analyzed in a digital signal processor (DSP) to locate the carrier frequency and to determine the locations of the undesired distortion components. However, this approach is often undesirable because the DSP and related circuitry increase the overall cost and complexity of the power amplifier.
Therefore, a need exists for a power amplifier that incorporates an IMD detector circuit that is relatively simple in design and that can be manufactured at a relatively low cost.